1. Field of the Invention
The present invention relates to an optical recording medium which records and/or reproduces information by a laser beam or the like, more specifically, to such an optical recording medium having a transparent dielectric layer as a protecting layer and/or an enhancing layer.
2. Description of the Related Art
Intensive research and development of optical recording mediums is underway, because of the high density and large information recording capacity of the mediums. Particularly, serious interest is shown in magneto-optical recording mediums, which are rewritable and thus can be applied in many fields. Nevertheless, although many materials and systems therefor have been proposed, there is still an urgent need for magneto-optical recording mediums which can be put to practical use.
Already proposed materials for magneto-optical recording mediums include TbFe disclosed in Japanese Unexamined Patent Publication (Kokai) No. 52-31703, and TbFeCo and DyFeCo, etc. disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-73746, and the like. Nevertheless, a further improvement of the reliability or durability and recording and reproducing characteristics is necessary before such magneto-optical recording mediums can be put to practical use. To this end, the provision of a transparent dielectric layer as a protecting and enhancing layer has been proposed, by which diffusion of a gas such as oxygen causing a deterioration of a recording material is prevented and an optical interference effect, i.e., multi-reflection of light, is utilized to increase the Kerr rotation angle. It is considered that nitrides, sulfides, fluorides, etc. of metals, such as Si.sub.3 N.sub.4, AlN, ZnS and MgF.sub.2, are preferable for the above transparent dielectric layer.
Among the above materials, the inventors investigated Si.sub.3 N.sub.4 and AlN, etc., which are considered to have an excellent resistance to the environment, and found that the deposition rates thereof are low and that the formed layers have large strains therein, and particularly, when a plurality of layers are formed, peeling, etc., of the considered layer along the grooves occur in an environment test, which is another problem of reliability or durability different to that of the resistance to oxidation of a recording layer. Also, the above dielectric materials have a refractive index of only around 2.0, limiting an increase of the Kerr rotation angle to around 0.5.degree. to 0.7.degree., which does not satisfy the specifications or requirements for practical mediums. Therefore, a further improvement of the reliability or durability and an increase of the Kerr rotation angle is required before these optical recording mediums can be put to practical use.